1. Field of the Invention
This invention relates to a magnetic shield system for electric cars or more in particular to a magnetic shield system for electric cars such that signal devices located along tracks are prevented from being erroneously operated by the magnetic fluxes produced from the electric car toward the track.
2. Description of the Prior Art
An electric car has a plurality of electrical devices in some form or other mounted on it. Such devices for a DC electric car include, for example, a DC motor for driving the car, a thyristor chopper for controlling the DC motor and a thyristor inverter or a commutatorless motor-generator as an auxiliary power supply device. An AC electric car, on the other hand, carries such electrical devices as an AC motor for driving the car and a phase control device for controlling the AC motor. These electric devices are connected by means of a plurality of electric wires for forming an electric circuit.
An electric car runs along a predetermined track, where provision is made for signal devices for transmitting and receiving vehicle information. These signal devices include, for example, a transmitter for transmitting predetermined signals to the rails of the track and a receiver located in spaced relation with the transmitter along the track for receiving the signals transmitted to the rails. When a car enters the track section between the transmitter and the receiver, the rails are short-circuited by the wheels of the car, cutting off the signals so far received by the receiver thereby to indicate the presence of the car in the particular track section.
When current is made to flow in the electric devices mounted on the car, on the other hand, magnetic fluxes are produced from the electric wires making up the electric paths and the electric devices themselves. If these magnetic fluxes have a direct effect on the signal devices, especially on the receiver, there is a danger of the receiver making a mistake by erroneously judging that a vehicle is absent between the transmitter and the receiver in spite of the short-circuited condition of the rails caused by the wheels of the car that have entered the track section between the transmitter and the receiver.
This kind of danger is not limited to the rails functioning as a signal transmission path. A signal device comprising a signal line along the track, which is in sliding contact with the pantagraph of the car to obtain the same relation as that between the rails and the wheels in the preceding case, is likewise exposed to a similar danger.